Motor drive system



April 20, 1948. E. w. KELLOGG MOTOR DRIVE SYSTEM Original Filed Deo.21., 1939 mtomeg Patented Apr. 20, 1948 Moron nmvE srs'rEM Edward W.Kellogg, Indianapolis, Ind., miglior to Radio Corporation of America, acorporation of Delaware Original application December 21, 1939, SerialNo. 310,385, now Patent September 15, 1942. tion April 30, 1942,

8 Claims.

This invention relates to motor drive systems such as are useful incorrelating the speeds of a plurality of separate motors under diilerentconditions of operation, and has for its principal object the provisionof an improved drive system and method of operation whereby variation inthe relation between the speeds of such motors is obviated or minimized.

This application is a, division of U. S. Patent 2,295,664, which issuedSeptember 15, 1942, on my application, Serial No. 310,365, filedDecember 2l, 1939.

Where it is necessary to drive several mechanisms which cannot bemechanically interconnected but which must run in strict synchronism, itis in most applications satisfactory to operate the several mechanismsby means of synchronous motors all supplied from the same power system.In other applications, it is necessary that the synchronism be preservedthroughout the operations of bringing the machines up to speed orstopping them. In other words, the several mechanisms are lockedtogether as if mechanically interconnected, and this interconnection isnot broken at any time. Where this is required, it is usual to employSelsyn motors. These have been used for many years and their principleof operation is well known. Briefly stated, a Selsyn motor isessentially the same as a wound rotor induction motor. Both the rotorand stator windings are preferably polyphase, but one or the other Imaybe single phase and the system still works. If the rotors of the severalmachines are connected together but not connected to any external sourceof power, and the stators are all supplied with alternating current froma common source, the rotors will seek a position at which the secondaryor rotor winding voltages are substantially balanced and little crosscurrent flows. If one of the rotors is forcibly turned, all of the otherrotors seek a new position of equilibrium, and this following of one bythe others continues for all speeds up to synchronism, although thetorque by which the several motors are held in step or synchronismbecomes weak as the rotor speed approaches the synchronous speed of thestator field. The frequency of the cross currents which hold themachines together approaches zero as synchronous speed is approached andlikewise the voltage which drives the currents through the windingsapproaches zero. Therefore, it is not practical to operate Selsyn motorsclose to synchronous speed. It is common to operate them at fromone-half to two-thirds of synchronous speed. It is equally possible touse the rotor wind- No. 2,295,664, dated Divided and this applica-Serial No. 441,149

2 ings as the primary Ito which the A'.-C. voltage is applied and tointerconnect the stators which then act as secondary.

An important example of the application oi Selsyn motors is in recordingsound for motion pictures. In this work it is necessary to thread filmrecords into a number of reproducing machines in such positions that thecontribution of each original record to the nnal recording in whichthese various original records are combined will occur at exactly theright time. The only practical Way to preserve this synchronism is tohave the machines so arranged that they will all come up to speedtogether, run together and slow down together.

Although Selsyn motors have been fairly satisfactory for this purpose,they are not above reproach. In certain applications, of which recordingsound is a striking example, a degree of speed constancy is required farin excess of that which is needed for most other purposes. Synchronousdriving systems, whether employing ordinary synchronous motors or Selsynmotors, are subject to hunting or an oscillatory action superimposed ontheir continuous rotation. This phenomenon has been recognized for yearsin synchronous motors and is well understood. 'I'he oscillations in manycases are not continuous and, after a disturbance, persist for only afew cycles, but they are started again by any little disturbance inpower supply or load. The cure for hunting lies in endowing the motorswith the property of damping out oscillations. Synchronous motors can begiven very powerful damping properties by use of pole face grids. Thesame expedient is not applicable to Selsyn motors and these areinherently more subject to oscillations and do not give as high an orderof speed constancy as the synchronous motors. Builders of Selsyn motorshave resorted to such expedients as coupling mechanical damping devicesto the motors which absorb energy from any oscillatory movement. Thismethod is helpful but heavy and cumbersome, and not as satisfactory asthe damping that is obtained in synchronous motors. It is the purpose ofmy invention to provide Selsyn motor systems with the superior dampingwhich characterizes synchronous motor systems, which at the same timepreserving the indispensable features of the Selsyn system of being ableto maintain absolute synchronism from start to stop of a run. For thispurpose, I employ Selsyn motors and a main driving motor exactly as inpresent Selsyn motor systems. As in present Selsyn systems. the drivingmotor drives a Selsyn unit which is called the master Selsyn, and issomewhat larger than the units attachedto the nlm phonographs, camerasand recorders which are t be operated in synchronlsm.

The several motors are locked together by applying a polyphase voltageto all oi' the primary windings, which are here assumed to be thestators, all of the rotors or secondarles being connected together, butno voltage being applied to the secondaries. When the machines are thuslocked together, the main motor is started, driving its master Selsyn,yand all of the other motors respond by rotating at their appropriaterelative speeds. There are cross currents between the secondary windingsof the machines, and these cross currents iiowing through the windingsreact with the rotating iields produced by the primary windings tosupply the torques required by the several motors. If the'chosenoperating speed is two-thirds of synchronous speed, a four-pole Selsynmotor will run 1200 R. P. M. (instead of 1800 R. P. M., which is thespeed at which a four-pole synchronous motor would run) Under theseconditions, the secondary currents have a frequency of cycles persecond.

Up to the point oi' reaching running speed, my system operates exactlyas an ordinary Selsyn driving system. From this point on, I provide thedriven machines with the superior steadiness and freedom from huntingcharacteristic of the true synchronous motor, and this may beaccomplished in the following manner: i

The polyphase power supply is removed from the primary winding and adirect current connection substituted. This change is accomplished sorapidly that none of the machines has an opportunity to drop out of stepor slip a pole during the moment of no power supply. The change can bemade, for example, in about one-tenth second.l When direct current'instead oi 60-cycle alternating current ows through the primary windingof the master Selsyn generator, the secondary current suddenly changesfrom. 20 cycles to 40 cycles. The magnetic :fields of the severalmotors, instead of rotating in space at synchronous speed, becomestationary. The l0-cycle current now produced by the main generatorreacts with this stationary eld in the motors to provide the requireddriving torque. Fortunately, no appreciable shift in phase takes placein the course of this transition. The generator voltage and the counterelectromotive forces of the motors were approximately balanced for the20-cycle circulating current and they are now also approximatelybalanced for the liO-cycle current. The entire system now operates nolonger as a Selsyn system but as a true synchronous generator andmotors. The entire transfer of power ls now accomplished by the iO-cyclecurrent.

At the end of the run, the reverse operation is performed. The directcurrent supplied to the primary winding is removed and the polyphasealternating current restored. The machines then come to rest together inaccordance with the standard Selsyn operation. Since the machines whichI employ mustserve during part oi the time as Selsyn motors, they maynot be provided with pole-face grids for damping. The reason for this iswell understood by electrical engineers. The grids must have very littlemotion with respect to the magnetic ileld produced by the winding in theother member of the motor. In a synchronous motor, the poles arecommonly in the rotor which is provided with pole-face grids, while thestator produces a eld which rotates at Syn 4 chronous speed. Since therotor also revolves at synchronous speed, there is no relative motionbetween the stator iield and the grids in the rotor.` In the case of theSelsyn, there is a rotating iield which is moving with respect totheprimary at synchronous speed and `with respect to the secondary atone-third synchronous speed. Poleface grids would have excessivecurrents induced in them and would destroy the effect of the rotatingiield, as well as produce a powerful torque tending to make the motorpull up to substantially full synchronous speed, acting as an inductionmotor. An effect exactly equivalent to the pole-face grids can beobtained in a synchronous motor by providing, in addition to the fieldwinding (normally excited by direct current), a sccond winding placed atelectrical degrees from the main ileld winding and short-circuited uponitself. The spatial relationship between these two windings is the sameas that between two windings of a polyphase alternating current winding.Thus, the polyphase primary winding of the Selsyn motor or generator maybe made to serve as a direct current field winding and a shortcircuiteddamping Winding. If the primary winding is two-phase, this means simplysupplying direct current to one of the polyphase windings andshort-circuiting the other. In the case of a three-phase winding, theidentical eiect may be obtained by connecting two of the .threeterminals together and using this as one terminal for the directcurrent, the third terminal serving as the other direct currentconnection. I have operated Selsyn motors in this manner and havedemonstrated the superior damping which may be obtained by thusoperating them as synchronous motors. The direct current required issubstantially the same as the maximum value of the alternating current,but since the only impedance to be overcome is the ohmic resistance ofthe conductors, a low voltage direct current supply, such as may beconveniently obtained from batteries, sufces.

For certain applications, it is important not only to prevent theslipping of a pole by any of the motors during a period oi.acceleration, but to provide an exact adjustment of phase position cithe several machines within a small number of degrees of arc. This, forexample, is the case in what is known as the background projtion systememployed in making motion pictures. The camera and the projector must beheld insynchronism to within a very small angle of tolerance and it maybe desirable to make adjustments of this angle. It is possible toproduce variations in the angle at which the machines are lockedtogether by slightly altering the distribution of direct current in thewindings. This may be done by small series resistors in the directcurrent supply.. `The exact running position may then be controlled bysetting these resistances at the proper values. This adjustment might benecessary to compensate for the eiect of the load on the motor so that,if the machines are set with certain relations at standstill, the exactrelation may be preserved during running. These resistancesneed not beso large as to materially impair the damping which depends on avirtually short-circuited winding. They only need to be comparable inmagnitude with the resistance of thewindings themselves in order tocontrol the current distribution.

Figures 1 to 3 illustrate a motor system involving the use .oraplurality of rotary converters, one of which is operated to control theothers. i

It is well known that rotary converters can be operated in parallel onboth the A. C. and D. C. sides. This has been made the basis of thesocalled A. C.D. C. interlock motor system, which has been commerciallyapplied to motion picture production, where it is desired to run severalmachines in strict synchronism. The A. C.D. C. interlock system has notbeen commercially applied to cases where it is desired to lock themachines together electrically at standstill and to bring them up instep. For this purpose, as has already been explained, Selsyn generatorsand motors have been employed. The reason that the A. C.D. C. motors,which from the standpoint of their synchronization are simplysynchronous A. C. machines, have not been used for interlocking fromstart is that neither they, nor any other synchronous generators, whenthey are turning over at very low speed as during starting, put outenough A. C. voltage to provide the necessary interchange of power tohold the machines together. For example, a generator which could supply100 volts across the A. C. terminals at full speed would develop onlyone volt when it is just starting and the speed is only 1% of fullspeed. The A. C.D. C. machine or rotary converter, however, can be madeto perform the function of a switching device whereby it can supply anappreciable voltage across its A. C. terminals at standstill as well asat extremely low speeds. If enough voltage is applied to the directcurrent brushes, for example, to send several times normal full loadcurrent through the armature windings, the slip rings will assumevoltages dependent on the positions of the taps in the windings to whichthey are connected. Thus, if the tap corresponding to slip ring A isconnected to the commutator bar directly under the positive brush, thisring will have substantially the same potential as the brush. If the taris half-way between brushes, the ring will be at mid-potential, or, ifthe tap is connected to the bar under the negative brush, the ring willbe at full negative potential. The windings serve as a double pathpotentiometer and the slip rings are connected to this potentiometer atpoints which vary relative to the brushes. Thus, at standstill, thethree rings, if the machine is wound for threephase, will assume certainpotentials dependent on the position of the armature and slip ring taps.If the armature is now rotated, no matter how slowly, the positions ofthe slip ring taps will shift from one brush to the other along thepotentiometer paths in such a manner as to cause a rise and fall andreversal of slip ring potential. In the absence of load, this slip ringpotential would be essentially a triangular or sharp peaked wave. Ifsome load is drawn from the slip rings, the wave shape will be changed,but it will still be alternating in character, and, although containingovertones or harmonics, the voltage will have a large component of thefundamental' frel quency. The relative phase of the voltages at thethree rings will depend on the positions of the taps to which they areconnected. If the taps are 120 electrical degrees apart, the voltageswill constitute a true three-phase system. Although movement of thearmature conductors through the magnetic field of the machine may be soslow that the induced voltage from this cause is negligible, the machinecan still supply a polyphase alternating voltage by pure resistancepotentiometer effect. The ratio of A. C. to D. C. voltage under theseconditions of operation will be substantially less than the ratio atfull speed or 6 normal operation, depending on the load, being of theorder of from one-half to one-third of the voltage which this normaloperation ratio would indicate.

If the slip rings of a machine, to winch voltage has been applied on theD. C. side as :lust described, are connected to the polyphase terminalsof a second machine, currents will flow between the two machines andthese currents, flowing through the windings of the second machine, willreact with the field thereof to lock the armature into a certainposition. If the first machine is at standstill, the second machine willrotate to a suitable position and remain there even though considerableforce is exerted to move it. If the armature of the first machine isrotated through a small angle, the distribution of currents in bothmachines changes, and the second machine moves to a new position. If themotion is continuous, the second machine rotates at the same speed atthe first. Although the currents in the second machine will, in general,be much weaker than those in the first machine, the latter may be madeseveral times full load current, under which conditions the currents inthe second machine will be abundantly adequate to cause the armature tostart up under load and rotate in synchronism with the first machine.This synchronous operation is maintained all the way from standstill tofull speed. As soon as the speed has reached a point where the inducedvoltage in the windings of the first machine becomes considerable incomparison with the applied D. C. voltage, the excessive current in thewindings of the first machine drops to a moderate value and the machinesare thereafter locked together by normal synchronous motor action. Thistransition from control of the current by resistance only, to control byelectromagnetically induced voltages, takes place at about the speed atwhich reactance in the motor windings begins to predominate over ohmicresistance.

If the D. C. voltage applied to the `brushes of the first machine isfrorna low voltage, low resistance source, the machine will not come upto full speed, but will continue to run at a low speed. If the D. C.voltage is from a source having a voltage suitable for full speedoperation, but if there is a suitable amount of resistance in the leadsto prevent excessive current at standstill, then the D. C. voltage willcontinue to rise as the machine comes up to speed, while the inputcurrent falls. During the entire acceleration, the output voltageisboosted somewhat by the resistance effect which at very low speed wasentirely responsible for the A. C- voltage.

It has been proposed by others to start several A. C.D. C. motorstogether and bring them up in complete interlock by impressing asuitable D. C. voltage across the brushes of each machine and dependingon the cross currents through the A. C. leads to hold the severalmachines in step. This method differs from the method of my invention inthat I supply most of the D. C. to one machine only. The cross currentswhich serve to tie 'the y machines together are not reduced ineffectiveness by impressing D. C. across both machines. The actualcurrent may be less but the current change which tends to retardwhichever machine is ahead or to accelerate whichever machine is behindis increased by applying D. C. voltage to the second machine. In otherwords, the strength of the interlock effect at low speeds is increasedby applying D. C, voltage to 'both machines. The dimculty with thismethod of bringing machines up to speed together lies in the fact thatthe direct current which flows in the windings of both machines exertsso powerful an accelerating torque that the interlock eifect is almostnegligible in comparison. The result is that, when several machines arestarted up in interlock in this manner, the amount of direct currentwhich is supplied to each machine must be carefully adiusted to its loadand, unless the direct current is kept to moderately small values, theentire system accelerates with great rapidity with the result that anydifferences in the inertia characteristics of the loads becomes magnied,tending to make the machine which must overcome the greater inertia,fall behind the machine which has less inertia load. In other words,both the friction load and the inertia loads need to be balanced. Thisresults in an extremely critical system.

AIn accordance with my invention, I sacrifice the slight increase ininterlocking torque which .might be obtained from supplying a largeamount of direct current to all of the machines, and depend for` myinterlocking polyphase current substantially entirely on the currentssupplied from the slip rings of one machine, a relatively large directcurrent being supplied to its brushes. The other machines then operateessentially' as synchronous motors, depending on the A. C. to supply anaccelerating torque rather than on direct current supplied directly totheir brushes. I may, however, supply a limited value of D. C. to thebrushes of the other motors, sufficient only lto overcome theirfrictional loads. By so doing, I reduce the amount of torque which mustbe supplied from the A. C. side `but do not permit the torque developedbythe D. C. in the windings to produce such a powerful forward torque asto require the A. C. interlock to exert an appreciable retarding torque.In order to produce enough A. C. output from the first machine whichserves as a master for the group, I may supply its brushes for a shortperiod with from three to six times normal full-load current. Thisresults in a corresponding increase in the A. C. voltage and currentavailable to fbe supplied to the other machines. This excessive D. C. inthe winding of the first machine results in the development of a verypowerful accelerating torque. Since, at the very low speeds, dependenceis placed on the potentiometer eiect for producing A. C. voltage, andelectromagnetically induced voltage plays a negligible part, it `wouldbe possible to prevent the excessive torque by temporarily weakening thefield of the master machine, the other fields being ,provided with fullexcitation from the start. expedient, however, is undesirable for thereasonf that it is desirable to develop the induced voltage as early aspossible in the course of acceleration. It might even be desirable tooverexcitei'thefelds of all of the machines during acceleration. Itherefore permitv the lexcessive torque tube-developed in the firstmachine but prevent it from accelerating toorapidly by some mechanicalmeans, as, for example, mounting a large-flywheel on the motor shaft.The moment of' inertia of the flywheel is sufilcient to prolong Ymachine during acceleration as it is after full expedients.' namely,limiting the direct current supplied to the armatures of the machineswhich are to be held in synchronism with the rst or master m-achine andreducing the rate of acceleration of the system, minimizes therequirements for interlocking torque and vthereby insures the adequacyof the torque developed bythe polyphase currents between the machines.

For some purposes, it is sufficient to insure that the machines belocked together at standstill, during acceleration and during the periodof full speed operation. If this is all that is required. thearrangements already described will sulce, except that on cutting offthe power supply the machine chosen as a master and provided with alarge flywheel would coast excessively. It is therefore desirable that abrake be applied to this machine which will bring it to rest in areasonable time. This brake can be automatically continue to flow, andto rise as the machine slows down to its original excessive value.Dependence must then be placed entirely on the brake to bring themachines to rest. As soon as the machines have come to rest, the D. C.supply to the master machine may be cut 01T.

The arrangement so far described will accomplish the requirements as setforth, namely, to

lock the machines together at standstill, bring them up to speed,operate at full speed and slow all machines down together in interlock.If it is desired to hold the machines in interlock over an appreciableperiod of time at standstill, it is desirable to provide a change ofconnections whereby the currents which hold the machines in fixedrelation will be supplied directly to the polyphase or slip ringconductors rather than to the brushes of any machine. This eliminatesany tendency for any of the machines to try to run. The fields of allmachines are maintained at full value, and D. C. voltage is applied tothe conductors -of the polyphase connections. This D. C. voltage may beapplied between any two of the three conductors or between one conductorand the other two, which may be connected together. When the stationarylocking current is applied in this manner, each machine receives its dueproportion of the current, which, in the arrangement I have shown inFigure 1, can be independently adjusted. When the switches are closedwhich apply D. C. to the A. C. leads, the machines will, in general,execute a small rotation, since the position at which they came to restwill be random, whereas the application of D. C. to the A. C. conductorswill force them to assume a specific position. This shift, however, willbe less than electric degrees and will not result in any slipping ofpoles. If desirable, an

auxiliary commutator, not illustrated, may be employed, which will lockthe machines at whatever position they have stopped, thus eliminatingany possibility of their being caught on "dead center and, therefore,not all executing identicalmovements. Alternatively, the brake may be'speedis reached. The combination of the two 75 arranged to bring themaster machine to final rest in a specified position of its revolution.In general, these reiinements will not be required.

Figure 2 illustrates in schematic form two A. C.D. C. motors, onedriving a sound recorder and the other a camera |02. The master motor|03v is provided with input conductors |04 connected to a D. C. supply|05 through the -switch box |06. The camera motor |01 is likewisesupplied with D. C. input conductors and switch, designated respectively|08 and |09. The A. C. conductors ||0 pass through a switch box by whichthey may be connected either to the D. C. supply |05 or to a polyphaseA. C. supply ||2. The flywheel is shown at ||3 with an electricallyoperated brake ||4 to bring the master motor to rest.

If a comparatively ylarge number of machines is to be operated in thismanner. it is desirable to make the master motor larger than the others,in order that the resistance of its windings may not too greatly limitthe current distributed to the cher machines. Such an arrangement isillustrated in Figure 1, where the master motor is not shown as drivingany of the machines, but it might do so if this appears to be desirable.Figure 3 shows another motor at |2| driving a recorder |0| and othermotors |22, |22, driving lrn phonographs |24, |24. The connections shownat represent multi-conductor cables or groups of cables through whicheach of the motors |2I, |22 is connected individually on both its A C.and D. C. sides to switch box |30, where suitable switches are providedto accomplish the several switching operations already described.

Typical switching arrangements are shown in Fig. 1 except that someobviously desirable switches are omitted for the sake of simplicity; as,for example, switches through which the elds are excited. A number ofswitches are shown in the drawing and it is to be understood that thesewould normally be operated as a unit, either by mechanically connectingthe various switch blades together or by providing a suitable group ofrelays, which operate from a main control and perform the switchingoperations.

For putting the system into operation, the master D. C. switch isclosed, thereby exy citing the iields of all of the machines. In orderto lock all of the machines together at standstill, the group ofswitches indicated at |4|, |42, |43, |44 and |45 would all be thrown tothe upper position. This, it will be noted, does not apply any D. C. tothe brushes of any of the machines, but supplies D. C. to the polyphaseconductors |41, |48, |49 and to the corresponding slip ring conductorsof the several motors. This D. C. for locking the machine together issupplied through resistors |52 to |55 inclusive, these resistors havingbeen adjusted to send current of suitable magnitude through eachmachine. Whenthe system is to be started, switches |4| to |45 inclusiveare thrown to the lower position which removes the D. C. voltage fromthe A. C. conductors |41, |48, |49 and establishes a through connectionof these conductors from each machine to all of the others. Switch |44connects the brushes of the master motor |20 to the D. C. supply,through the comparatively low resistance |51 which permit a largecurrent to iiow when the machines are at standstill. Switch |45 connectsthe motor |60, which may represent motor |01, |2| or |22 of the otherfigures, to the D. C. supply through resistances |58 which are ofrelatively high value and permit only enough D. C. to iiow through thebrushes of motor |00 to overcome load Iriction. The motors will now comeup to speed in the manner already described. When full speed is reached,it may be desirable to synchronize all oi the machines with a main(iO-cycle polyphase power supply, the frequency of which is accuratelymaintained from the power house. For this purpose, switch |6| is shown.Synchronization would be performed in the manner that is common whenlarge synchronous motors or alternators are brought up to speed by someauxiliary power and then synchronized with others. As one method ofindicating the time to close switch ISI, the synchronizing lamps |52 areindicated. There are well-known devices for automatically synchronizingmachines with the line and any such automatic system may be employed.For stopping the system, switch |6| would be opened. The brake would beapplied to the motor |20 and, as soon as it has been brought to rest,switches |4| to |45 inclusive would be thrown to the upper positionwhereby the machines are locked together for standstill. Resistances |51are shown as variable, whereby the speed of the entire system may becontrolled, as for example, to facilitate synchronizing with thepolyphase supply. The. current'l limiting resistances |51, |51 and |58,|58 are shown as divided substantially equally between the positive andnegative connections. If either the positive or the negative brushes areconnected together through'a low resistance connection, the master motor|20 will transfer a combination of D. C. and A. C. to the othermachines, and the D. C. may produce too much forward torque. Moreover,the D. C. which thus flows between the machine robs the A. C. powertransfer and thus weakens the interlock.

I claim as my invention: p

1. In a drive system for the synchronous operation from standstill tofull speed of a. plurality of electrodynamic machines each provided withpolyphase alternating current terminals and with direct currentterminals, the combination of means for supplying direct current to saidalternating current terminals for interlocking said machines atstandstill, means for interconnecting said alternating currentterminals, means for supplying a relatively large direct current to thedirect current terminals of one of said machines and a relatively smalldirect current to the direct current terminals of the other of saidmachines for starting said machines, and means for sychronizing saidmachines with an alternating current supply source.

2. In a drive system for the synchronous operation from standstill tofull speed 0f a plurality of electrodynamic machines each provided withpolyphase alternating current terminals and with direct currentterminals, the combination of switching means for supplying directcurrent to said polyphase terminals for locking said machines togetherat standstill and for interconnecting said polyphase terminals duringthe acceleration of said machines, and means for supplying relativelylarge and small values of direct current to said direct currentterminals during said acceleration.

3. In a drive system for the synchronous operation from standstill tofull speed of a plurality of electrodynamic machines each provided withpolyphase alternating current terminals and with direct currentterminals, the combination of switching means for supplying directcurrent to said polyphase terminals for locking said mall chinestogether at standstill and ior interconnecting said polyphase terminalsduring acceleration of said machines, means for supplying relativelylarge and small values or direct current to said direct currentterminals during said acceleration, alternating current supplyterminals, and means for connecting said alternating current supplyterminals to the alternating current terminals of said machines fornormal operation i said machines.

4. In a drive system ior the synchronous operation from standstill tofull speed oi` a plurality of electrodynamic machines each provided withpolyphase alternating current lterminals and with direct currentterminals, the combination oi switching means for supplying directcurrent to said polyphase terminals for locking said machines togetherat standstill and for interconnecting said polyphase terminals duringacceleration o! said machines, means for supplying relatively large andsmall values of direct current to said direct current terminals foraccelerating said machines, and braking means for decelerating saidmachines.

5. In a drive system for the synchronous operation from standstill tofull speed of a plurality of electrodynamic machines each including awinding provided with polyphase alternat ing current terminals and withdirect current supply terminals, the combination ot means iorinterconnecting the alternating terminals of a plurality oi.' saidmachines, means for supplying direct current to the direct currentterminals oi one oi said machines. and means for preventing rapidacceleration o! said machine.

6. In a drive system for the synchronous operation from standstill tofull speed of a plurality of electrodynamic machines eachv including awinding provided with polyphase alternating current terminals and withdirect current supply 4f, 2,132,630

l2 rapid acceleration of said machine, and mechanical means for bringingsaid machine to rest.

'1. In the operation of a drive ysystem for the synchronous operationfrom standstill to full speed of a plurality oi electrodynamic machines,each provided with polyphase alternating current terminals and withdirect current terminals, the method which includes supplying a directcurrent to one of said machines large in comparison with the normaloperating current ci said machine, retarding acceleration oi saidmachine, and decelerating said machine to standstill while continuing tosupply direct current to said machine.

8. In the operation of a. drive system for the synchronous operationfrom standstill to full speed of a plurality of electrodynamic machineseach including a winding provided with polyphase alternating currentterminals and with direct current terminals, the method which includes'supplying direct current to said polyphase terminals for interlockingsaid machines, interconnecting said polyphase terminals for acceleratingsaid machines, supplying to the direct curterminals, the combination ofmeans for interconnecting the alternating terminals oi a plurality oi'said machines, means for supplying direct current to the direct currentterminals of one oi said machines, means for preventing rent terminalsof one of said machines a direct current greater than that applied tothe corresponding direct current terminals of the other oi saidmachines, and applying a retarding force to the machine supplied withsaid greater direct current.

EDWARD W. KELLOGG.

ssrmnhcns man The following references are of record in the ille of thispatent:

UNIT@ STA'I'ES PATENTS Number Name Date 845,725 Alchele Feb. 26, 1907Kengon Dec. 5, 1939 FOREIGN PATENTS Number Country Date 193,458 GreatBritain Feb. 26, 1923

